Conventionally, in a car navigation system, to improve the detection accuracy of the current position and heading angle of the vehicle, a contrivance is made for obtaining on a horizontal plane the variance of the heading angle of vehicle movement and distance of vehicle movement (velocity) more accurately.
For example, as for a vehicle position detecting device disclosed in Patent Document 1, to eliminate a problem in that a yaw rate detected by an angular velocity sensor has an error when there is a lateral inclination of the body or a lateral road slope during a turn of a vehicle, the vehicle position detecting device estimates a yaw rate error using a roll angle, and carries out, by correcting the heading angle error involved in the yaw rate error, map matching considering a position calculation or heading angle error of the vehicle. In this case, the roll angle is calculated according to a prescribed equation using the angular velocity sensor and a distance sensor.
In addition, as for a rotation plane maintaining angular velocity sensor disclosed in Patent Document 2, to eliminate a problem in that a yaw rate an angular velocity sensor detects reduces when there is a body inclination (pitch angle) or road slope in the direction of vehicle movement of the body while the vehicle is moving on a slope, the rotation plane maintaining angular velocity sensor corrects the yaw rate using a road slope angle. In this case, the road slope angle is calculated using the angular velocity sensor and a distance sensor.
Besides, as for an onboard navigation system disclosed in Patent Document 3, to eliminate a problem of causing an error because the distance of vehicle movement (velocity) calculated from the pulse signal during traveling a slope, which the distance sensor outputs in accordance with the number of revolutions of the wheels of the vehicle, becomes longer than the distance (velocity) projected onto a horizontal plane, the onboard navigation system corrects the distance of vehicle movement (velocity) on the horizontal plane using the road slope angle. Furthermore, in the same manner as the technique disclosed in Patent Document 2, it corrects the yaw rate using the road slope angle. In this case, the road slope angle is calculated from the axial accelerations detected by a 3-axis acceleration sensor and the acceleration of the vehicle measured by a distance sensor.
In addition, as for an angular velocity correcting device disclosed in Patent Document 4, to eliminate a problem in that when the vehicle is moving a slope during a turn, the lateral acceleration component of the body is superimposed onto the acceleration in the direction of vehicle movement of the body detected by an acceleration sensor, the angular velocity correcting device calculates the pitch angle by subtracting the lateral acceleration component of the body from the acceleration detected by the acceleration sensor and corrects the yaw rate on the basis of the pitch angle. In this case, the lateral acceleration component of the body is calculated using the velocity and the yaw rate.
Furthermore, as for a road geometry measuring device disclosed in Patent Document 5, to eliminate the problem in that the lateral acceleration component of the body is superimposed onto the acceleration in the direction of vehicle movement of the body detected by an acceleration sensor in the same manner as the technique disclosed in Patent Document 4, the road geometry measuring device calculates the road slope angle and road bank angle (angle of the lateral inclination of the road) by removing the lateral acceleration component of the body from the acceleration detected by the acceleration sensor. In addition, it detects the angle (pitch angle or roll angle) of the body in the traveling or lateral direction of the body because of the effect of the acceleration or deceleration in the direction of vehicle movement of the body or the lateral acceleration of the body with a vehicle height sensor, and extracts the true road slope angle and body bank angle. In this case, centrifugal force is calculated using the velocity measured by the distance sensor, the yaw rate detected by the angular velocity sensor, and the distance between the rotation axis of the body and the fixed position of the acceleration sensor.    Patent Document 1: Japanese Patent Laid-Open No. 05-018765/1993.    Patent Document 2: Japanese Patent Laid-Open No. 06-324066/1994.    Patent Document 3: Japanese Patent Laid-Open No. 09-42979/1997.    Patent Document 4: Japanese Patent Laid-Open No. 2005-140627.    Patent Document 5: Japanese Patent No. 3576789.
FIG. 1 shows a coordinate system (Body Frame: B coordinate system) fixed to the body according to a right-hand orthogonal 3-axis system consisting of the direction of vehicle movement (XB axis) and the lateral direction (YB axis) of the body and the vertical direction (ZB axis) to an XB-YB plane. The angular velocity sensor and the acceleration sensor attached to the casing of the car navigation system in such a manner that the sensor detection axis agrees with one of the XB axis, YB axis and ZB axis detect the angular velocity (roll rate ωB-roll, pitch rate ωB-pitch and yaw rate ωB-yaw) about the sensor detection axes and the acceleration (ABX, ABY and ABZ) in the sensor detection axis directions. Incidentally, as for the angular velocity about the sensor detection axes, the clockwise rotation is assumed to be a positive direction and its unit is “rad/s”.
FIG. 2 shows a coordinate system (Computer Frame; C coordinate system) with respect to a horizontal plane. For example, when the vehicle is moving along a slope, its body inclines with respect to the horizontal plane, and the sensor detection axis tilts by an angle called an attitude angle of the body (in particular, a body pitch angle θC-pitch and a body roll angle θC-roll) so that the sensor is affected by the remaining axes. In particular, as for the acceleration sensor, it is necessary for the signal processing to consider not only the effect of the gravity, but also the effect of driving operation (operation of the accelerator, brake and wheel) and centrifugal force.
FIG. 3 is a diagram for explaining relationships between a sensor mounted angle δθyaw in the yaw direction and a sensor mounted angle δθpitch in the pitch direction and the attitude angle with respect to the horizontal plane. Unless the sensor mounted angle in the yaw direction or pitch direction is zero, the sensor detection axis is tilted fixedly. Thus, the sensor is always affected by the remaining axes. Incidentally, the subscript “B” denotes a parameter of the B coordinate system, and likewise the subscript “C” a parameter of the C coordinate system.
Considering the foregoing explanation, the detection accuracy of the position and heading angle of the vehicle is determined by the sensor signal processing ability for the following (1)-(3).
(1) A detecting method of the inclination of the sensor detection axis with respect to the horizontal plane (particularly when using the acceleration sensor).
(2) The effect of the inclination of the sensor detection axis with respect to the horizontal plane has on the angular velocity and acceleration about the sensor detection axis.
(3) Correction of the drift error produced in the sensor.
On the other hand, concerning the foregoing (1) and (2), the techniques disclosed in Patent Document 1-Patent Document 3 carry out only correction of the distance and angular velocity and positioning calculation by limiting to one-sided effect of the sensor output brought about by the inclination of the sensor detection axis caused by the road slope without considering the effect of the body lateral acceleration. For example, when the vehicle makes a turn while running along a slope or makes a high-speed horizontal turn, since the output signal of the acceleration sensor changed by the effect of the lateral acceleration is detected as the effect of the road slope angle, the measuring error of the attitude angle (pitch angle in particular) of the vehicle increases. Accordingly, it is very likely that improper correction of the yaw rate detected by the angular velocity sensor or of the distance of vehicle movement (velocity) detected by the distance sensor occurs, which will bring about reduction in the detection accuracy of the position and heading angle of the vehicle.
On the other hand, as to the foregoing (2), the techniques disclosed in Patent Document 4 and Patent Document 5 detect the angle of inclination and correct the angular velocity taking account of the body lateral acceleration, and thus consider more multifaceted effect of the sensor output than the techniques disclosed in Patent Document 1-Patent Document 3. However, since it does not seem that the foregoing (1) is considered, when the casing of the car navigation system is set in the body in such a manner as to rotate about one of the axes of the B coordinate system, the sensor detection axis is always in a condition of being inclined even if the vehicle is moving on a horizontal plane. For example, when the casing of the car navigation system is set in such a manner as to be rotated in the yaw direction, although the XB axis acceleration sensor outputs the composite acceleration of the XB axis acceleration and the YB axis acceleration, the techniques disclosed in Patent Document 4 and Patent Document 5 do not consider the fact. Accordingly, they incorrectly detect variations in the XB axis acceleration due to the YB axis acceleration as a road slope angle during a turn, and make improper correction of the yaw rate detected by the angular velocity sensor. Thus, this is very likely to bring about reduction in the detection accuracy of the position and heading angle of the vehicle.
In addition, concerning any one of the techniques of Patent Document 1-Patent Document 5, since drift correction, that of the acceleration sensor in particular, is not carried out properly with respect to the foregoing (3), the detection error of the attitude angle of the vehicle (the pitch angle and roll angle in particular) increases when the drift occurs. Thus, it is very likely to bring about reduction in the detection accuracy of the position and heading angle of the vehicle.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a car navigation system capable of measuring the position and heading angle of the vehicle at high accuracy.